In order to provide a more compact format for identifying mixtures of refrigerants in the following discussions, mixtures of refrigerants will be listed in the form of:R-ABC/DEF/GHI (N0/N1/N2)                orR-ABC/DEF/GHI (N0-N0′/N1-N1′/N2-N2′)which represents a mixture of refrigerants (fluids) R-ABC, R-DEF, and R-GHI where N0, N1, and N2 are the weight percentages of each component fluid. The second form is similar, but specifies ranges of weight percentages of each of the component fluids, with the total being 100 percent. For this application, the following Table 1 discloses some refrigerant R-numbers referenced in this discussion along with their chemical names.        
TABLE 1R-numberChemical nameR-12dichlorodifluoromethaneR-22chlorodifluoromethaneR-32difluoromethaneR-124chlorotetrafluoroethaneR-134a1,1,1,2-tetrafluoroethaneR-1341,1,2,2-tetrafluoroethaneR-142b1-chloro-1,1-difluoroethaneR-600n-butane (n-C4H10)R-600aisobutane (i-C4H10) or i-butaneR-601n-pentane (n-C5H12)R-601aisopentane (i-C5H12) or i-pentaneR-1270propyleneR-227ea1,1,1,2,3,3,3-heptafluoropropaneR-125pentafluoroethaneR-290propaneR-C270cyclopropaneR-E170dimethyl ether (DME)R-152a1,1-difluoroethaneR-115chloropentafluoroethaneR-143a1,1,1-trifluoroethaneR-218octafluoropropaneR-500R-12/152a (73.8/26.2)R-502R-22/115 (48.8/51.2)R-413AR-218/134a/600a (9/88/3)R-414AR-22/142b/124/600a (51.0/16.5/28.5/4.0)R-404AR-125/143a/134a (44/52/4)R-407CR-32/125/134a (23/25/52)R-417AR-125/134a/600 (46.6/50.0/3.4)
The U.S. Environmental Protection Agency and other world scientific bodies have determined that refrigerants comprised of chlorofluorocarbons (CFCs) cause harm to the Earth's stratospheric ozone layer after being released into the atmosphere. Hydrochlorofluorocarbons (HCFCs), which are chlorofluorocarbons containing one or more hydrogen atoms, also cause damage to the ozone layer, although much less so, and often twenty times less than CFCs. Various rules, regulations, protocols and treaties in the world have phased out CFCs, or are now doing so. HCFCs are being allowed as “transistion” substances between CFCs and zero ozone depletion alternatives under development. HCFCs are also under phase outs, but on a much longer time scale than the CFCs, with the final HCFC phaseout being in the year 2030. Certain countries may phase out HCFCs earlier than 2030, and certain categories of use (e.g., production of new equipment containing HCFCs) may be phased out earlier, as well, while other categories of use (service fluids for repair of existing equipment) may be maintained until 2030.
Several non-ozone depleting refrigerants have already been developed in the prior art. However, all of them have one or more drawbacks. The main drawback is that refrigerants comprised of hydrofluorocarbons (HFCs), or mixtures thereof, do not mix (are not miscible in) mineral oils used for the earlier CFC and HCFC refrigerants (e.g., CFC-12 and HCFC-22, R-502, R-500). R-407C, comprised of R-32/125/134a (23/25/52), closely matches the pressure temperature curve of HCFC-22. However, it requires ester (POE) oil and is totally non-miscible in mineral oil. Ester oils are far more expensive and are less stable than mineral oils. Ester oils also are far more hygroscopic than mineral oils, so moisture can enter a refrigeration system much easier during manufacture or service than it can with mineral oils. This moisture is extremely damaging, and causes the refrigerants to slowly hydrolyze and decompose into hydrofluoric acid (HF) and other components leading to early system failures.
Steel is a catalyst that can make some ester refrigeration oils decompose back to their components of formation, namely alcohols and fatty acids. Ester oil manufacturers often add proprietary “passivators” to prevent the breakdown of their ester oils. Passivators can sometimes wear out or be consumed in long term operation (i.e., years), thus leading to oil failure. Mineral (or alkylbenzene) oils are inherently stable over the long term in properly operating refrigeration systems, often lasting 30-40 years or more.
U.S. Pat. No. 5,688,432 to Pearson teaches an R-22 substitute consisting of a mixture of R-125, R-134a, and a hydrocarbon selected from the group consisting of propane and isobutane. The '432 patent's Summary of Invention section mentions “(iii) a hydrocarbon selected from isobutane, propane and mixtures thereof.” All examples and claims only show isobutane for the hydrocarbon, no propane or “mixtures thereof” are taught for the use as the hydrocarbon in this mixture. The '432 patent claims greater than zero and a maximum 11 weight percent of isobutane and that the resulting mixture is “nonflammable”. The definition of “flammable” when applied to pass a UL2182 test to qualify for an ASHRAE classification (flammability) of group 1 will only permit about 3 weight percent maximum isobutane in these mixtures. The UL2182 standard requires fractionation of the mixture at various temperatures. Vapor and liquid samples taken during the fractionations are then flammability tested to make sure no regions of flammability occur. Before ignition, the gas/air mixtures are preheated to 100° C. (for the “as formulated” mixture) and 60° C. (for the worst case fractionations). In addition, “worst case manufacturing tolerances,” where the nominal mixture has all flammable components increased by 0.5 weight percent and nonflammable components decreased by 0.5 weight percent, are used instead of the nominal mixture. Nowhere does the '432 patent teach breaking up the hydrocarbons between isobutane and propane to better distribute the flammable components among the nonflammable components in order allow the weight percentages of the flammable components to be increased. A formulation of R-125, R-134a, which contained 8-11 weight percent isobutane would return mineral oil very well to the compressor and indeed may be nonflammable “as formulated,” however, it certainly becomes flammable during fractionation and elevated ignition preheat temperatures as specified under UL2182, thus limiting the maximum isobutane to about 3 weight percent, which is not enough to return mineral oil in all cases. It may suffice in window A/C units with short suction lines, but may not work in systems with long or uphill refrigerant lines, such as found in rooftop units or units running “unloaded” with low suction gas velocities.
Also, in the prior art is R-417A (Rhodia ISCEON 59), comprised of R-125/134a/600 (46.6/50.0/3.4), which contains a hydrocarbon in an attempt to carry the mineral oil. While this refrigerant will work in systems that circulate very little mineral oil, or have short refrigerant return lines to the compressor, such as window A/C units, it might not always be able to return the mineral oil to the compressor in long piping runs as often found in super market or rooftop air conditioner installations.
In 1995, Applicant built an “oil miscibility” test stand used in the development of R-414A [R-22/142b/124/600a (51/16.5/28.5/4)] (U.S. Pat. No. 5,151,207 and other co-pending applications). This simulated worst case oil return for developing R-12 substitutes at the time. Applicant ran a mixture of 5 weight percent isobutane, 95 weight percent R134a in mineral oil and noted that there was almost zero oil return. (R-134a has zero miscibility in mineral oil). Also the “liquid line” side of the system with this mixture looked like “milk,” white opaque, in the sight glass, which signified that an “oil dispersion” of tiny droplets of mineral oil still existed in the high pressure side of this system, a sign that all of the circulating oil was not dissolving into the refrigerant mixture. If all the oil had dissolved in the refrigerant, the sight glass would have been clear. R-417A operates at significantly lower pressures than does R-22, thereby causing a capacity loss on the order of 15-30 weight percent, especially in systems with fixed refrigerant metering devices (capillary tubes).
R-417A is ASHRAE safety classified as A1 and is covered by UK patent GB2327427 to Roberts (and now U.S. Pat. No. 6,428,720). Roberts teaches that flammability during fractionation can be reduced by substituting n-butane for isobutane, for n-butane boils at about 31° F. verses about 10° F. for isobutane at 1 atmosphere (ATM) pressure. Roberts goes on to teach and claim that adding higher boiling point hydrocarbons (from C4 and greater, excluding isobutane) also reduce vapor flammability during fractionation (e.g., isopentane, pentane, etc.).
Another mixture in the prior art, very similar to R-417A, is available as a commercial product designated RS-44. RS-44 consists of R-125/134a/600/601a (50/47/2.5/.5) and is disclosed in International Application No. PCT/GB00/03725, International Publication No. WO 01/23493 A1, of inventors Richard Powell, et al. In a similar fashion, Powell et al. also teach the addition of higher order hydrocarbons (n-butane, isopentane in RS-44) to reduce flammability (lower HC vapor pressure component). Roberts (R-417A) and Powell et al. (RS-44) both teach that their mixtures of gasses do properly return mineral oil to the compressor. However, nothing is taught about mineral oil return under partial load conditions or with commercial systems that have “unloaders” that reduce return gas velocity in the suction line. An unloader is a method by which some commercial HVAC systems use to modulate compressor capacity to track the refrigeration load. Unloaders may include turning off one or more compressors in a rack of paralleled compressors, disabling one or more cylinders in a piston compressor, usually by forcing some valves not to operate. Vanes or valves may be used to restrict the suction line and newer systems may use variable frequency (inverters) drives to vary the speed of the compressor motor. All unloader methods cause a reduction (commonly to ½) of the suction gas velocity. Reducing the suction gas velocity makes it more difficult to return mineral oil to the compressor. No prior art has been found that addresses mineral oil return under reduced suction gas velocities with chlorine free nonflammable alternative refrigerants. Unlike R-417A, RS-44 closely matches R-22 in pressures and capacity.
It has also been noted that industry in the US “requires” that refrigerants for almost all applications must have passed a UL2182 flammability test and must have an ASHRAE safety designation of “A1” (nonflammable, even after worst case fractionation, and lower toxicity group).
Adding high boiling point hydrocarbons to refrigerant blends to pass the UL flammability test seems to work, but even n-butane, boiling at +31° F. (at one ATM), will largely tend to stay dissolved in the bulk of the oil in the compressor crankcase (which is often at 5 ATM pressure in R-22 class systems) and may not vaporize in the suction line or the compressor crankcase, so much of it will not circulate in the refrigerant stream to the evaporator where it is needed to help return the mineral oil to the compressor. Isopentane and n-pentane are even worse in this respect. Moreover, if one has a “leaky” system, which needs recharging often, n-butane, isopentane, or pentane will tend to just build up in the oil with each successive recharge, and may eventually thin the oil out enough to interfere with proper compressor lubrication. One may then have a “flammability” problem when replacing the failed compressor due to the excessive amount of pentanes/butanes remaining in the compressor oil.
The boiling points (in order) of isobutane, n-butane, isopentane, n-pentane at 1 ATM are about 11° F., 31° F., 82° F., and 97° F. With conventional hermetic piston or scroll compressors, the “crankcase” operates at the low side (suction line) pressure, which is typically about 60 PSIG (75 PSIA) on an R-22 air conditioning system. At this pressure, the boiling points of the above hydrocarbons become (in order) 102° F., 124° F., 185° F., and 201° F. Crankcase temperatures on the above compressors are typically in the 90° F. thru 110° F. range, high enough such that isobutane and maybe even some n-butane will probably stay boiled out of the crankcase oil. Boiling at 185° F. and 201° F., the pentanes are much more likely to stay dissolved in the mineral oil in a crankcase operating at 90-110° F., thus they will tend to just thin out the crankcase oil a little instead of circulating with the refrigerant. Rolling piston and rotary vane compressors, often called “rotaries,” which are commonly used in small to medium sized window A/C units have the crankcase at the discharge (high) pressure side, so the pentanes are even MORE likely to just stay dissolved in the crankcase oil than on compressors with the crankcase on the suction side.
Published US Patent Application No. 2002/0050583 to Caron et al., discloses mixtures of R-125, R-134a, and dimethyl ether (DME) to act as a replacement for R-22 in mineral oil. Caron et al. teaches that 3.5 to 25 weight percent of DME is needed to provide proper return of the mineral oil. Nothing is mentioned about flammability. Caron et al. also has claims in the range of 5 to 8 weight percent DME (preferred) as being needed to return mineral oil properly. They do not teach about the extent of mineral oil return, whether it be short runs such as in a window A/C unit, or long runs such as in a rooftop or supermarket unit with tough oil return problems. In Applicant's tests, 5 weight percent DME in this mixture would definitely be flammable and not pass a UL2182 test by itself. Although being flammable under a UL2182 test, 5 to 8 weight percent of DME would definitely offer excellent mineral oil return in an R-22 replacement, even under reduced suction gas velocities (from unloaders) based on Applicant's experiences in mineral oil return. If flammability (probably even “weak” flammability) can be tolerated, Caron et al.'s mixtures should be excellent refrigerants. DME boils at about −12.7° F. at one ATM (67.5° F. at 5 ATM), which is well below the typical crankcase temperatures, therefore, the DME will not become “trapped” in the crankcase oil and will circulate with the refrigerant where it will aid in the mineral oil return.
Also, some refrigerants such as R-413A R-218/134a/600a (9/88/3), contain a “perfluorocarbon” (R-218, octafluoropropane). Perfluorocarbons, while legal in refrigerants outside the US, are highly frowned upon by the US EPA due to their extremely long atmospheric lifetimes (thousands of years) and their very high global warming potentials, and consequently they are usually not approved for refrigerant use in the US.